1. Field of the Invention
The present invention relates to a Diesel particulate filter (hereinafter, abbreviated as DPF), an additive used for a fuel oil or a lubricating oil composition for a Diesel engine having DPF and a fuel oil comprising the additive. More particularly, the present invention relates to the DPF, the additive, the fuel oil and the lubricating oil composition which improve the combustion property of a particulate matter (hereinafter, abbreviated as PM) trapped with a filter of DPF, enable combustion of PM at a low temperature with stability, improve the efficiency of removal of PM and increase the life of DPF.
2. Description of the Related Arts
Diesel engine vehicles have advantages in that the fuel efficiency is more excellent than that of gasoline engine vehicles, the amount of carbon dioxide is reduced effectively and cost of gas oil used as the fuel oil is lower than that of gasoline. However, PM contained in the combustion gas discharged from Diesel engine vehicles is recently regarded as a great problem on the environmental pollution. PM is minute residues of combustion of fuel oils such as soot and it is known that PM adversely affects the respiratory system when PM is taken into the human body. Therefore, the reduction in the amount of PM in the discharged gas is the greatest problem for Diesel engine vehicles.
In Japan, in accordance with the new regulation which is to be enforced by 2005, every manufacturer of Diesel engine vehicles must reduce the amount of the discharged PM to ⅓ of the amount in accordance with the present regulation. In the municipality of Tokyo, it is under study that installation of DPF should be enforced to all Diesel engine vehicles by 2003. It is highly possible that the same regulation is enforced in other municipalities, also.
Under the above circumstances, the development of efficient and practical DPF has been actively conducted and several types of DPF have been proposed. Specifically, (1) DPF of the alternate regeneration type, (2) DPF of the continuous regeneration type by oxidation with NO2, (3) DPF of the continuous regeneration type by the catalytic oxidation and (4) DPF of the intermittent regeneration type have been proposed.
In DPF (1) of the alternate regeneration type, two filter units each having a metal net heater and a protective metal net disposed on both faces of a non-woven fabric of a carbon silicate fiber are used and the trapping and the regeneration are conducted by switching the operations alternately between the two filter units. This DPF can be used for the currently used gas oil having a great content of sulfur. However, this DPF has drawbacks in that a great electric current is required for combustion of PM and a dynamo having a great capacity must be installed for this purpose alone and, moreover, that the filter may be damaged due to rapid combustion.
In DPF (2) of the continuous regeneration type by oxidation with NO2, a porous ceramic filter of cordierite having the wall-flow honeycomb structure is used as the filter. This is an apparatus of the continuous regeneration type in which NOx is oxidized into NO2 by an oxidation catalyst placed at the upstream of the filter and PM trapped with the filter is burned at a lower temperature by utilizing the strong oxidizing ability of NO2. However, this DPF has a drawback in that, since the activity of the oxidation catalyst is decreased by sulfur in the exhaust gas and the function of the catalyst is not sufficiently exhibited, the sulfur content in the fuel oil must be decreased and the application of this DPF to the currently used gas oil is difficult.
In DPF (3) of the continuous regeneration type by the catalytic oxidation, a porous ceramic filter of cordierite having the wall-flow honeycomb structure coated with two types of metal catalysts is used. This is an apparatus of the continuous regeneration type in which no heating apparatus such as a heater is necessary and PM trapped by the filter is burned by the working of the metal catalysts alone. This DPF can be applied to the currently used gas oil since the metal catalysts are relatively less susceptible to the effect of sulfur in the exhaust gas. However, this DPF is susceptible to the effect of phosphorus and the smaller the sulfur content, the more excellent the performance. Moreover, the application of this DPF to vehicles and passenger cars driven at a low speed for a long time is difficult since it is necessary that the vehicle be driven under a condition such that the temperature of the exhaust gas is 300° C. or higher for a specific fraction of time in the entire driving time.
In DPF (4) of the intermittent regeneration type, a porous ceramic filter of carbon silicate having the wall-flow honey-comb type is used as the filter for trapping PM. For the regeneration, the temperature of the exhaust gas is elevated by injecting the fuel. The temperature is further elevated by oxidation of hydrocarbons and carbon dioxide by the working of the oxidation catalyst and PM is burned. In this DPF, a cerium compound is added to the fuel oil and the amount of formed PM is decreased. This DPF can be applied to the currently used gas oil. However, this DPF has drawbacks in that an apparatus for the source of electricity for regeneration is necessary and that cerium oxide remains after the combustion of PM and is accumulated in the filter and the life of DPF is decreased.
As described above, DPF currently under development are not always satisfactory.
Industrially, it is important that the function of DPF is effectively exhibited and the life is increased. It is important for this purpose that the amount of the discharged PM is decreased and PM is burned at a temperature as low as possible.
It is also important that clogging of the filter is prevented as much as possible while PM is burned at a temperature as low as possible. When the filter is clogged, the efficiently of the Diesel engine is decreased due to an increase in the backpressure. The clogging of the filter are caused by clogging with residues of the combustion of PM and by clogging with ashes from the lubricating oil.
As the method for decreasing the amount of the discharged PM and for lowering the temperature of combustion of PM, (i) a method comprising lowering the temperature of combustion of PM by adding a cerium compound to the fuel oil as described in DPF (4) of the intermittent regeneration type and (ii) a method comprising decreasing the amount of the discharged PM by adding a salt of an alkaline earth metal or an iron compound to the fuel oil, have been known.
However, method (i) comprising adding a cerium compound has a drawback in that cerium compounds remain after the combustion and is accumulated on the filter and the life of DPF is decreased although the amount of PM can be decreased when the cerium compound is added in a relatively great amount. Method (ii) comprising adding a salt of an alkaline earth metal or an iron compound has a drawback in that residues of the combustion are accumulated on the filer and the life of DPF is decreased in a similar manner to that in method (i) described above.